The invention relates to an optical glass fiber with a primary coating, which, at least partially, consists of organo-polysiloxanes, having alkyl groups bound to silicon, and containing acrylic acid ester groups. The organo-polysiloxanes are liquid at the application temperature and are polymerizable through high-energy radiation.
Optical glass fibers are coated with synthetic materials at the time of their production. These synthetic coatings function to protect the glass fibers and to maintain their physical, in particular optical, properties against the mechanical and chemical influences of the environment. In order to meet these complex needs, the glass fibers are generally coated with two different types of coatings.
The first coating, i.e. the inner cladding layer (primary coating) in general is soft and rubbery elastic. It serves to maintain the mechanical strength inherent in the glass fibers and to protect them against microbends and attenuation. The primary coating should therefore have a glass transition temperature of less than -40.degree. C. and its mechanical properties within the application temperature range of -40.degree. to +80.degree. C. should only change as little as possible, so that within these temperature limits a constant attenuation response of the coated glass fibers is ensured. The modulus of elasticity of the coating should be less than or equal to 10 N/mm.sup.2. In many instances the index of refraction of the coating substance must be higher than that of the outer layer of the glass fiber. Given an index of refraction n.sub.D.sup.25 of quartz glass between 1.45 and 1.46, then for the inner coating substance an index of refraction of greater than 1.46 within the application temperature range is required.
The second coating, (i.e., the outer synthetic cladding) functions to protect the glass fiber covered with the first synthetic material coating against mechanical and chemical effects of the environment, in particular against the effects of moisture and against mechanical abrasion. The second coating is therefore mechanically harder than the first.
The coating substances are applied onto the glass fibers while they are being manufactured. Since the drawing process can take place at rates of greater than 5 m/s and since curable synthetic materials or synthetic preparations are used as coating substances, curing of the synthetic materials or synthetic preparations must take place in less than 1 second, and preferably in less than 0.5 seconds.
Many types of two-fold coatings are already known. These are primarily UV-curable urethane acrylates which form the primary as well as the secondary coating. Such coatings with urethane acrylates are described for example in DE-AS No. 34 37 531 and EP-OS No. 0 111 280. These materials, however, do not possess the required complex properties discussed above. Within the last few years organo-polysiloxanes modified with acrylic acid ester groups have proven useful. Given the appropriate structure, these polymers yield desirable soft-elastic coatings after curing with UV-radiation within the required time of less than 0.5 seconds.
A glass fiber with such primary cladding is described in EP- A1-0 155 051. The glass fiber is provided with a cladding having a refractive number which is higher than that of the outer layer of the glass fiber. The cladding is formed of a curable synthetic composition containing a copolymer, which has as monomeric units dimethylsiloxane and at least one siloxane from the group methylphenylsiloxane and diphenylsiloxane. The copolymer itself is a compound having the following configuration: ##STR2## in which R.sub.1 and R.sub.8 are alkyl, aryl, and acrylate-containing alkyl groups, R.sub.2, R.sub.3, R.sub.4, R.sub.6, and R.sub.7 are alkyl and aryl groups, and R.sub.5 is an acrylate-containing organic group. The mean molecular weight of the polymer molecules lies between 1000 and 1 000 000, and the average fractions of the monomeric units per polymer molecule lie withing the following limits:
p is greater than or equal to 0.005 and is less than or equal to 0.995 PA1 q is greater than or equal to 0 and is less than or equal to 0.995 PA1 r is greater than or equal to 0 and is less than or equal to 0.49 PA1 s is greater than or equal to 0 and is less than or equal to 0.1, PA1 R.sup.2 is a linear-chain, branched or cyclic alkyl residue with 5 to 18 carbon atoms, which can possibly be substituted with a halogen, PA1 R.sup.3 is an aralkyl of the following structure: ##STR4## with R.sup.4 is hydrogen and/or alkyl with 1 to 12 carbon atoms and/or halogen, and PA1 Q is a multiple acrylic and/or methacrylic acid ester of a polyol bound through the oxygen atom of a hydroxyl group reduced by the hydrogen atom to silicon, the polyol being from the group consisting of pentaerythritol, trimethylolethane, trimethylol-propane, and glycerin, as well as their dimers and/or oxyalkylene derivatives with 1 to 10 oxyethylene and/or oxypropylene units, where PA1 a is greater than or equal to 0.9 and b plus c is less than or equal to 0.9 PA1 b, c are 0 to 1.1, PA1 b plus c is greater than or equal to 0.1, PA1 .SIGMA.a,b,c are 1.8 to 2.2, and PA1 d is 0.001 to 1.6,
where (q+2r) is greater than or equal to 0.005 and is less than or equal to 0.955 and the monomeric units can be distributed in the molecule block-wise, alternating, or in any given way, and where the copolymer contains at least two acrylate groups per molecule.
The acrylate-containing alkyl group is a group of the formula --R.sub.9 OCOCH.dbd.CH.sub.2, where R.sub.9 is an alkylene residue, for example the group --(CH.sub.2).sub.n --with n greater than or equal to 1. Consequently, the coating materials are made functional with monoacrylate groups, which are bound to the siloxane chain through a Si--C bond.
Published international patent application WO No. 84/00424 describes primary coating materials comprised of silicon acrylates having a molecular weight of 500 to 5000 and 2 to 6 monoacrylate groups. These monoacrylate groups are preferably bound through Si--C bonds to the siloxane chain. Silicon acrylates with a Si--O--C bond are considered to be less suitable as primary coating materials since a potential hydrolytic degradation of the coating materials, through changing mechanical properties, would lead to a change of the attenuation response and, consequently, to unsatisfactory long-term behavior of the optical fiber.
The state of the art organo-polysiloxanes modified with acrylic acid ester groups are manufactured by preparing organo-polysiloxanes having reactive groups and subsequently converting them with organic compounds which contain acrylate groups. In addition to Si bound methyl groups, the organo-polysiloxanes have a minimum quantity of phenyl groups bound to Si atoms in order to meet the required refractive number. The organo-polysiloxanes are, therefore, prepared through cohydrolysis and cocondensation of silanes carrying methyl and phenyl groups. As unavoidable by-products varying quantities of cyclic compounds form which cannot be separated by distillation. They can only be removed from the resulting organo- polysiloxanes with particular difficulty. These cyclic compounds, therefore, after conversion with the compounds containing the acrylate groups, are contained in the coating substance as an inert component. Since these compounds are not tied into the polymeric matrix in the curing process, they can, in the course of time, migrate from the coating substance. However, when this occurs the elastic properties of the cured coating substance are changed. Furthermore, improper adhesion of the second coating layer can occur, which leads to partial or complete separation of the second coating.
Another disadvantage of organo-polysiloxanes with Si-bonded phenyl substituents is that with increasing content of phenyl groups the freezing temperature of the cured coating materials is shifted toward higher temperatures (i.e., &gt;-40.degree. C.). Such organo-polysiloxanes are not suitable for coating optical glass fibers, since they would lead to marked changes of the elastic properties within the application temperature range of optical glass fibers.
Accordingly, it is an object of the invention to provide an optical glass fiber with a primary coating of organo- polysiloxanes which has a high refractive index and a glass transition temperature below -40.degree. C. but which does not have the disadvantages described above and which is free of inert components capable of migration.
Another object of the invention is to provide a primary coating which has good bonding with the second coating despite the fact that the second coating is not based on organo-polysiloxanes and consists of polyurethanes.
Another object of the invention is to provide a soft-elastic primary coating which demonstrates improved compatibility with organic coating materials and which is easily applied.